Pulsed metal or metal halide lamps for photocopying applications

ABSTRACT

A low pressure metal or metal halide vapor lamp for photocopying applications. In a preferred embodiment, a low pressure sodium vapor lamp is operated in a pulsed mode, providing illumination of increased brightness and of a color (yellow) which is optimum for providing a copy which is an accurate representation of the original.

United States Patent Hug Oct. 21, 1975 [54] PULSED METAL OR METAL HALIDE3,596,237 7/1971 Barber et a1. 313/201 X LAMPS O PHOTOCOPYING 3,619,71611/1971 Spira et al 315/244 APPLICATIONS William F. Hug, Pasadena,Calif.

Xerox Corporation, Stamford, Conn.

Filed: May 6, 1974 Appl. No.: 467,343

Inventor:

Assignee:

US. Cl 315/241 R; 313/187; 313/228; 315/115; 315/243; 315/289; 355/69Int. Cl. G03B 27/54; HOSB 41/34 Field of Search 313/187, 228, 229, 197,313/198, 201; 355/67, 69, 71-; 315/234, 241 S, 241 R, 243, 244, 289,335, 358, 115

References Cited UNITED STATES PATENTS Gunto et al 355/67 x PrimaryExaminerJames W. Lawrence Assistant ExaminerE. R. La Roche Attorney,Agent, or Firm-James .l. Ralabate; Terry J. Anderson; Irving Keschner Alow pressure metal or metal halide vapor lamp for photocopyingapplications. In a preferred embodiment, a low pressure sodium vaporlamp is operated in a pulsed mode, providing illumination of increasedbrightness and of a color (yellow) which is optimum for providing a copywhich is an accurate representation of the original.

ABSTRACT 7 Claims, 3 Drawing Figures US. Patent Oct. 21, 1975 Sheet 1of2 3,914,649

PuLsD' MODE SPECTRAL RADI ANCE cONfiNous MODE l l I 4000 5000 6000 7000WAVE LENGTH (ANGSTROMS) FIG. 3

US. Patent Oct. 21, 1975 Sheet 2 of2 3,914,649

PULSED METAL OR METAL HALIDE LAMPS FOR PHOTOCOPYING APPLICATIONSBACKGROUND OF THE INVENTION In the xerographic process as described inU.S. Pat. No. 2,297,691, a base plate of relatively low electricalresistance such as metal, etc., having a photoconductive insulatingsurface coated thereon is electrostatically charged in the dark. Thecharged coating is then exposed to a light image. The charges leak offrapidly in the base plate in proportion to the intensity of light towhich any given area is exposed, the charge being substantially retainedin non-exposed areas. After exposure, the coating is contacted withelectrostatic materials which adhere to the remaining charges to form apowder image corresponding to the latent electrostatic image remainingafter exposure. The powder image then can be transferred to a sheet oftransfer material resulting in a positive or negative print, as the casemay be. Since dissipation of the surface electrostatic charge isproportional to the intensity of the impinging radiation, light sourcesof uniform and sufficient intensity must be provided so that thephotoconductive insulator can be properly exposed.

Low pressure metal halide lamps are a near optimum illumination sourcefor photocopiers producing black and white output copies from black andwhite and multi-colored originals.

With respect to line copy, the optimum goal of any black and whitephotocopying apparatus is to make the image areas on the copy as blackas possible. In other words, one would like a minimum of energyreflected from the image areas of the original while reflecting amaximum from the background region. Obviously, it is impossible to copyall colored backgrounds as white while concurrently copying all coloredimages as black.

From prior experience, it appears that most colors that are utilized asimages on an original tend to be located at the extremes of the visiblespectrum, i.e., blues and reds, whereas yellow, for example, is seldomutilized for images. Colored backgrounds are pastel (desaturated) andcan usually be considered as tinted white paper which may be explainedin part on well known principles of physiological optics (photopticvision).

It then follows that the optimum light source for photocopying apparatusproducing black and white output copies from black and white andmulti-colored originals produces yellow light whereby black and redswill copy as black, while concurrently most common colored papers haveconsiderable reflectance in yellow (it should be noted that the use ofthe yellow exposure lamps obviously necessitates a yellow sensitivephotoreceptor). However, the typical prior art photocopying apparatusutilizes aperture fluorescent lamps which generate colored light.

Low pressure sodium lamps represent a commercially available yellowlight source. Present commercial sodium lamps, such as thosemanufactured by N. V. Phillips, have several disadvantages forphotocopying applications associated therewith.

The principal problem is that a long warm-up period is required beforethe lamp may be operated at its optimum efficiency, i.e., at anoperating temperature of 260C. For example, whereas unassistedfluorescent lamps require only a matter of seconds to reach peakradiance, unassisted sodium lamps require several minutes. Additionalproblems arise if the sodium lamp is operated in a continuous mode (mostphotocopying apparatus operate with the illumination lamp continuouslyenergized). For example, the photoreceptor may fatigue when it iscontinuously flooded with light produced by said sodium lamp, the heatgenerated by the sodium lamp may harm the photoreceptor, and continuousoperation of the lamp may add to the cost of a customers electricalbill. Although the equipment may be devised to protect the photoreceptorfrom the heat and light (when copying is not in progress) generated bythe lamp, additional apparatus to effectuate this protection would addto the cost of the photocopier and the complexity thereof. Further, thebrightness (illuminance) ofa continuously operated low pressure sodiumlamp is less than desirable.

SUMMARY OF THE PRESENT INVENTION The present invention providesapparatus for operating a low pressure metal halide lamp in a pulsedmode for use in photocopying apparatus. In particular, a low pressuresodium vapor lamp is operated in a, noncontinuous, or pulsed modewhereby an original is illuminated only when a copy is desired. Thepulsed mode of operation eliminates the need for mechanical apertureswhich would be required to prevent illumination on the photoreceptorand/or operator between copies, increases lamp brightness by increasingthe current density over nominal value for d.c. operation, and increasespotential copy speeds because of increased lamp brightness and totaloutput. The use of sodium vapor lamps, in particular, enables copies tobe made of most originals, the copies being an accurate representationof the original.

It is an object of the present invention to provide apparatus forutilization in a photocopying apparatus wherein a low pressure metal ormetal halide lamp is utilized, in a pulsed mode of operation, as theillumination source.

It is a further object of the present invention to provide apparatus forutilization in photocopying apparatus wherein a low pressure sodium lampis utilized, in a pulsed mode of operation, as the illumination source,maximum lamp brightness and efficiency being obtained.

It is still a further object of the present invention to provideapparatus for providing a low pressure sodium vapor lampoperable in apulsed mode of operation which provides a spectral output which isparticularly useful in photocopying applications.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thepresent invention, as well as otherobjects and further features thereof,reference is made to the following description which is to be read inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram showing a low pressure metal halide vaporlamp connected in operating relation to an electric circuit with adirect current power source in combination with a trigger circuit and astandby heating source;

FIG. 2 illustrates an electrostatic photocopying apparatus in which thepresent invention may be utilized; and

FIG. 3 is a graph depicting the brightness levels obtained when the lampis operated in the continuous and pulsed modes.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, anenvelope of material capable of being formed into a sealed container towithstand evacuation to partial vacuums and capable of transmittingdesired wavelengths of radiation generated by the metal or metal halidegaseous medium within the envelope is shown. Lead-in wires 11 areembedded in the envelope 10, each lead-in wire bearing an electrode 12and 13 in spaced-apart relationship.

The anode 12 is connected by electrical conductor 14 through an inductor2 to the positive terminal of capacitor 16 that may be charged through acharging resistance 19 from an energy source, such as battery 15, whenswitch 17 is closed. Theother, or cathode, electrode 13 is connected tothe negative terminal of capacitor 16. I

When switch 17 is closed, the voltage to which the capacitor 16 ischarged by battery may, depending upon various characteristics of lamp1, in and of itself be sufficient to effect a discharge through thegaseous medium within the envelope 10 between the anode electrode 12 andthe cathode electrode 13. This initial voltage required to operate thelamp is referred to as the breakdown voltage and is usually greater thanlater lamp operating voltages because of increased ionization andelectrical conductivity of the gaseous medium within envelope 10 afterthe lamp has been operated for a time. A triggering circuit includingsource 20 and external winding 21 provides an alternative technique forigniting lamp 1. For example, capacitor 16 may be charged as describedhereinabove to a voltage below the breakdown voltage for the particularconditions of flashlamp 1. (Note that the operation of a lamp in a 7mode which is non-continuous, i.e., alternately on and off, is generallyreferred to in the art as a flashlamp.) The flashlamp 1 may then betriggered by means of trigger circuit 20 and 21 by transmitting a'pulsefrom source 20 to external winding 21 to cause partial ionization of thegaseous medium within envelope 10 making the medium conductive enough topermit the voltage stored in capacitor 16 to become discharged throughthe gaseous medium from the anode 12 to the cathode 13, therebyproducing high-intensity radiation of a wavelength dependent upon thegaseous medium utilized. Using this alternate technique of triggering,once the triggering pulse is completed on the winding 21, the conductionpath between the electrodes 12 and 13 continues until the energy storedin capacitor 16 is dissipated. Capacitor 16 is then charged via battery15 as described hereinabove. Therefore, if source 20 comprises arepetitive source of voltage which has the proper amplitude to triggerthe ionization of lamp 1, a technique is provided whereby the firing ofthe flashlamp may be controlled in accordance with predeterminedconditions.

Lamp l, in accordance with the teachings of the present invention, is ametal 'or metal halide vapor lamp, and in the preferred embodiment,comprises a low pressure sodium vapor lamp. When the gas is ionized, asdescribed hereinabove, an extremely intense flash of actinic light ofrelatively short duration, such as for a period of less than 100microseconds, is generated.

Since low pressure sodium is utilized, the spectral output ispredominantly in the 5,900 A range and corresponds to yellow light.

A heating voltage 22 is provided, via diode 23, to heat the winding 21to maintain the envelope 1 at an elevated temperature (i.e.,approximately 260C) which causes the vapor pressure of the sodium(approximately 0.01 Torr) to be correspondingly optimized foroptimumsodium line output, i.e., spectral output is primarily centered about5,900 A. In the apparatus shown in FIG. 1, the resistance wire 21 isused both to heat the envelope 1 and maintain the sodium vapor pressureat its optimum condition and to provide the trigger electrode to inducea volumetric breakdown in'the sodium vapor and to allow capacitor 16 todump its energy through lamp 1, thereby producing the illumination pulseas described hereinabove.

In a typical embodiment, the pulse from source 20 is of a width fromabout 30 microseconds to about 1,000 microseconds and the radiationproduced by said pulse has an energy density range from about 10" toabout 10 joules per square centimeter.

Other techniques may be utilized to raise the lamp temperature such asapplying a transparent conductive coating to the lamp envelope or byutilizing a heating resistance within the lamp envelope, as shown inU.S. Pat. No. 2,755,400.

Low pressure sodium vapor lamps and their operating characteristics aredescribed in the article by W. Elenbaas et al, Improvements inLow-Pressure Sodium Vapor Lamps, Illumination Engineering, Volume 64,Page 94, February, 1969. Particular temperatures, operating pressures,glass envelope composition, etc. described in the article will similarlycharacterize the low pressure sodium vapor lamp in accordance with theteachings of the present invention. Further, the sodium vapor lampdisclosed in U.S. Pat. No. 3,400,288 may easily be adapted to the pulsedmode of operation in accordance with the teachings of the presentinvention.

Low pressure sodium vapor lamps are commercially available, for example,from North American Philips Corporation, New York, New York(manufactured by N. V. Philips, Eindhoven, the Netherlands).

In addition to the sodium vapor lamp described hereinabove, othermaterials, such as thalium, thalium iodide and potassium, may beutilized as the gaseous medium albeit sodium is preferred for thereasons set forth hereinabove.

FIG. 2 schematically illustrates apparatus for electrostaticallyphotocopying documents, or originals, using the xerographic process andthe subject matter of the present invention. The sodium vapor lampapparatus described in FIG. 1 (like elements labeled with identicalreference numerals) is positioned above original 18 from which copiesare to be made. In the embodiment illustrated, endless belt 20 having aconductive base 31 and a coating 32 of photoconductive material, whichis selected to be rendered conductive by the illumination generated bylamp 1 is arranged to run on spaced drums 40, 42, and 44 underneathdocument 18. Although not shown in the figure, drums 40, 42, and 44 aredriven in tandem by an appropriate drive motor.

In operation, the photoconductive belt 30 which essentially comprises alayer made of selenium on an alloy thereof overlying a conductivesubstrate is initially charged at charging station 50. The chargingstation comprises a generally well known corona charging device as shownin the prior xerographic art, for example. The belt 30, shown in anendless belt configuration, is driven in the direction of arrow 51. Whenthe lamp 1 is energized in the manner described hereinabove (i.e., whena copy is to be made), the illumination generated thereby (the sodiumspectral line in the preferred embodiment).illuminates' document 18. It.

should be noted that the apparatus of FIG. 2 assumes that the original18 is a transparency since the illumination will pass therethrough toexpose photoconductive layer 32. Obviously, illumination lamp 1 can beposi tioned below original 16 if the original is opaque, the generatedlight being reflected therefrom. In any event, the portions of thedocument 18' corresponding to image areas or dark areas are absorbed'andthe background or transparent areas illumination are passed through todischarge the appropriate portions of the belt 32. The belt is advancedto a development station 60 whereat a housing 62 contains a charge ofelectroscopic toner particles 64 and. a roll 66 having a brush 68 on itssurface. As roll 66 rotates, the brush 68 passes through the tonerparticles and then across the surface of belt 30, distributing the tonerparticles over the surface of the belt. The toner particles adhere tothe belt in areas containing a residual charge, but not in the unment16. Alternate development techniques may be utilized, such as powdercloud development as described, for example, in US. Pat. No. 2,701,764.

At station 70, this image is transferred to image receiving web 72. Web72 is drawn from supply roll 74 and is guided in contact with belt for ashort distance by guide rolls 76 and 78. Transfer of the toner particlesconstituting the developed image may be aided by an appropriateelectrical field or by charging of the web 72 by corona chargingelectrode 70', as is well understood in the art. After the image istransferred to the web 72, the web maybe passed through a heater 75 tofuse the toner particles to the web, and the web is guided by roll 79 toa delivery station.

Since the photoconductive layer 32 is to be reused for a subsequentimaging cycle, after the image transfer operation residual tonerparticles are removed from the surface of belt 30 by brush 82 at station80. The photoconductive layer 32 may then be exposed to an illuminationsource, to erase any residual electrostatic image. Before returning tothe optical exposure station, the surface of the photoconductor isexposed to a general corona discharge at station 50, to provide auniform electrostatic charge over the photoconductive layer 32 andthereby enable electrostatic optical recording of an image of thedocument 18.

If a multiple number of copies of original 18 are to be made, the sameoperation is repeated. If a copy of a different original is desired,the'original 18 is replaced with the different original.

FIG. 3 is a graph illustrating the difference in brightness levels(spectral radiance) between a low pressure sodium vapor lamp operated inthe continuous (a.c. or d.c.) mode as opposed to the lamp being operatedin the pulsed mode, the increased brightness which is obtained from alow pressure sodium vapor lamp operated in the pulsed mode being clearlyshown.

While the invention has been described with reference to its preferredembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teaching of the inventionwithout departing from its essential teachings.

What is claimed is:

1. Apparatus for forming a latent electrostatic charge pattern on aphotoconductive insulating medium, said latent electrostatic chargepattern corresponding to the radiation pattern projected from aninformation bearing member comprising:

a charged photoconductive insulating member,

a' low pressure metal vapor source for generating radiation of apredetermined spectral line, said information bearing member beinginterposed in the optical path between said metal vapor source and saidcharged photoconductive insulating member, and

. means for operating said low pressure metal vapor source in a pulsedmode whereby pulses of radiation are emitted therefrom, a radiationpulse exposing said information bearing member to' selectively dissipatethe charge on the surface of said photoconductive insulating member inaccordance with the intensity of the radiation pulse projected from saidinformation bearing medium to form said latent electrostatic chargepattern.

2. The apparatus as defined in claim I wherein said low pressure metalvapor source comprises a metal halide vapor source.

3. The apparatus as defined in claim 1 wherein said low pressure metalvapor source comprises a sodium vapor lamp.

4. The apparatus as defined in claim 3 wherein said low pressure sodiumvapor lamp comprises:

an envelope having spaced-apart electrodes and low pressure sodium vaportherein,

means for coupling a voltage of an amplitude below the breakdown voltageof said sodium vapor across said lamp electrodes, and

means for applying a voltage to said lamp of an amplitude sufficient tocause said sodium vapor to breakdown whereby a radiation pulse isemitted therefrom.

5. The apparatus as defined in claim 4 further including means forheating the sodium vapor within said envelope.

6. The apparatus as defined in claim 5 wherein each radiation pulse isgenerated for a period in the range from about 30 microseconds to about1,000 microseconds and in the energy density range from about 10 toabout 10 joules per square centimeter.

7. The apparatus as defined in claim 6 wherein the pressure of thesodium vapor within said envelope is approximately 0.01 Torr.

1. APPARATUS FOR FORMING A LATENT ELECTROSTATIC CHARGE PATTERN ON APHOTOCONDUCTIVE INSULATING MEDIUM, SAID LATENT ELECTROSTATIC CHARGEPATTERN CORRESPONDING TO THE RADIATION PATTERN PROJECTED FROM ANINFORMATION BEARING MEMBER COMPRISING: A CHARGED PHOTOCONDUCTIVEINSULATING MEMBER, A LOW PRESSURE METAL VAPOR SOURCE FOR GENERATINGRADIATION OF A PREDETERMINED SPECTRAL LINE, SAID INFORMATION BEARINGMEMBER BEING INTERPOSED IN THE OPTICAL PATH BETWEEN SAID METAL VAPORSOURCE AND SAID CHARGED PHOTOCONDUCTIVE INSULATION MEMBER, AND MEANS FOROPERATING SAID LOW PRESSURE METAL VAPOR SOURCE IN A PULSED MODE WHEREBYPLUSES OF RADIATION ARE EMITTED THEREFORM, A RADIATION PULSE EXPOSINGSAID INFORMATION BEARING MEMBER TO SELECTIVELY DISSIPATE TUBE CHARGE ONTHE SURFACE OF SAID PHOTOCONDUCTIVE INSULATING MEMBER IN ACCORDANCE WITHTHE INTENSITY OF THE RADIATION PULSE PROJECTED FROM SAID INFORMATIONBEARING MEDIUM TO FORM SAID LATENT ELECTROSTATIC CHARGE PATTERN.
 2. Theapparatus as defined in claim 1 wherein said low pressure metal vaporsource comprises a metal halide vapor source.
 3. The apparatus asdefined in claim 1 wherein said low pressure metal vapor sourcecomprises a sodium vapor lamp.
 4. The apparatus as defined in claim 3wherein said low pressure sodium vapor lamp comprises: an envelopehaving spaced-apart electrodes and low pressure sodium vapor therein,means for coupling a voltage of an amplitude below the breakdown voltageof said sodium vapor across said lamp electrodes, and means for applyinga voltage to said lamp of an amplitude sufficient to cause said sodiumvapor to breakdown whereby a radiation pulse is emitted therefrom. 5.The apparatus as defined in claim 4 further including means for heatingthe sodium vapor within said envelOpe.
 6. The apparatus as defined inclaim 5 wherein each radiation pulse is generated for a period in therange from about 30 microseconds to about 1,000 microseconds and in theenergy density range from about 10 4 to about 10 2 joules per squarecentimeter.
 7. The apparatus as defined in claim 6 wherein the pressureof the sodium vapor within said envelope is approximately 0.01 Torr.